1. Field of the Invention
The present invention relates to a method of manufacturing an annealed wafer obtained by subjecting a silicon single crystal wafer to high-temperature heat treatment.
2. Description of the Related Art
Annealed wafers obtained by subjecting silicon single crystal wafers to heat treatment at a high temperature have been widely used as high-quality substrates for manufacturing semiconductor devices.
Generally, a silicon single crystal wafer has a defect called a void defect. Here, the “void defect” refers to a defect caused as a result of aggregation of introduced atomic-level vacancies at an interface between a silicon melt and a crystal during crystal growth. Among the “void defects”, the one exposed in a surface of the silicon single crystal wafer is called a COP (Crystal Originated Particle).
By subjecting the silicon single crystal wafer to heat treatment at a high temperature of, for example, not less than 1100° C., the above “void defects” shrink or are eliminated, and the defects in the surface of the wafer can be reduced. Further, by adding nitrogen to the silicon single crystal wafer, the size of the “void defects” is reduced, and the “void defects” shrink or are eliminated readily by heat treatment.
On the other hand, the silicon single crystal wafer also contains oxygen precipitates. “Oxygen precipitates” are generated when oxygen mixed into the silicon melt from a quartz crucible blends into the crystal in a concentration more than a saturating concentration, and is aggregated by the heat treatment described above. The “oxygen precipitates” in the surface of wafer are reduced by the heat treatment described above. The “oxygen precipitates” can be generated inside the wafer by adding nitrogen to the silicon single crystal wafer.
By adding nitrogen to the silicon single crystal wafer and subjecting the silicon single crystal wafer to heat treatment at a high temperature as described above, it is possible to form a substantially denuded region with a thickness of about several microns in the surface of the wafer, and to leave appropriate “void defects” and “oxygen precipitates” inside the wafer. That is, a high-quality silicon single crystal wafer can be obtained.
As one of control parameters when pulling a silicon crystal by the Czochralski method, “V/G” has been known. Here, “V (mm/min)” represents the pulling rate of the silicon crystal, and “G (° C./mm)” represents the temperature gradient in an axial direction when the silicon crystal is grown.
Japanese Patent Laid-Open Application No. 2007-176732 describes an invention which focuses on the relationship between “V/G” described above and nitrogen concentration, and attempts to obtain an annealed wafer in which precipitation of oxygen after annealing is sufficiently high and oxygen precipitates in a wafer surface have a uniform density. In addition, Japanese Patent Laid-Open Application No. 2010-155748 describes a method of manufacturing an annealed wafer which adds nitrogen and hydrogen into a furnace for pulling a silicon single crystal to control “V/G”, in order to obtain a denuded zone with a thickness of not less than 10 μm in a surface of a silicon single crystal wafer, in the case where a high concentration of nitrogen (not less than 5E14 atoms/cm3) is contained.
In contrast, Japanese Patent Laid-Open Application No. 2006-312576 describes a method of manufacturing a silicon single crystal which does not focus on “V/G” but adds a gas of a substance containing hydrogen atoms to an atmosphere gas within the growth apparatus and further dopes nitrogen or/and carbon into the crystal. In addition, Japanese Patent Laid-Open Application No. 2000-281491 describes a method of manufacturing a silicon single crystal which grows the silicon single crystal by the Czochralski method, by continuously introducing hydrogen gas at 3% by volume to 0.1 ppm into an atmosphere.
It has been found that, if a substrate cut out from a crystal having a nitrogen within a certain range is used as a substrate of an annealed wafer, there arises a problem in the quality of the annealed wafer. Specifically, if the nitrogen concentration is less than 1E15 atoms/cm3 (that is, less than 1×1015 atoms/cm3), the number of voids remaining in the wafer surface after annealing is increased. It has also been found that, if the nitrogen concentration exceeds 4E15 atoms/cm3, Time Dependent Dielectric Breakdown (TDDB) characteristics of an oxide film formed on the annealed wafer are deteriorated.
The reason for the increase in the number of voids remaining in the wafer surface after annealing when the nitrogen concentration is less than 1E15 atoms/cm3 is because a crystal having a nitrogen concentration of less than 1E15 atoms/cm3 has voids which are much larger (not less than 0.5 μm) than conventionally known voids with a size of about 0.2 μm, and which have a low density (not more than 1E4/cm3), and they are not fully eliminated by annealing.
On the other hand, the reason for the deterioration of the TDDB characteristics of the oxide film formed on the annealed wafer when the nitrogen concentration exceeds 4E15 atoms/cm3 is because a crystal pulled under a condition in which nitrogen is present within a high range has grown-in defects, which have not been known and are different from voids, and they are not eliminated by annealing.
Therefore, the range of the nitrogen concentration is limited to not less than 1E15 atoms/cm3 and not more than 4E15 atoms/cm3. If the range of the nitrogen concentration is narrow, a top portion of the crystal having a low nitrogen concentration and a bottom portion of the crystal having a high nitrogen concentration cannot be used as a product wafer, and thus crystal yield is reduced, making it difficult to manufacture the annealed wafer at low cost.
With the methods of Japanese Patent Laid-Open Application No. 2007-176732, Japanese Patent Laid-Open Application No. 2010-155748, and Japanese Patent Laid-Open Application No. 2000-281491, an annealed wafer using a substrate cut out from a crystal having a nitrogen concentration of less than 1E15 atoms/cm3 has many residual voids, and an annealed wafer using a substrate cut out from a crystal having a nitrogen concentration of more than 4E15 atoms/cm3 has deteriorated TDDB characteristics. Therefore, the range of the nitrogen concentration is limited to not less than 1E15 atoms/cm3 and not more than 4E15 atoms/cm3. Since the range of the nitrogen concentration is limited to a narrow range, crystal yield is reduced, making it difficult to manufacture the annealed wafer at low cost.
In contrast, in Japanese Patent :Laid-Open Application No. 2006-312576, it is necessary to grow a crystal under a condition in which no grown-in defects are present, which requires that the crystal should be grown with V/G as a crystal growth parameter described later being limited to a very narrow range. To grow the crystal in such a narrow range, it is necessary to add special ingenuity to a crystal growth apparatus. For example, Japanese Patent Laid-Open Application No. 2006-312576 discloses making the temperature gradient of a crystal peripheral portion lower than the temperature gradient of a crystal central portion during crystal growth. However, such ingenuity results in a reduction in crystal growth rate (for example, according to Japanese Patent Laid-Open Application No. 2006-312576, the pulling rate is about 0.3 to 0.6 mm/min). As a result, the productivity of the crystal is reduced, and the manufacturing cost of the silicon wafers is increased in total, although there is no need for annealing. Further, if the range of V/G is limited to a narrow range, this results in an increase in an area of the crystal which is out of the V/G range due to variations in crystal growth, causing a reduction in crystal yield.